Kachina Bridge hikers, in Natural Bridges National Monument, near Blanding, Utah, USA. White Canyon Creek has cut Kachina Bridge with a span of 192 feet through white Permian sandstone of the Cedar Mesa Formation. Kachina is named for rock art on the bridge that resembles symbols commonly used on kachina dolls. Manganese-rich desert varnish requires thousands of years to coat a rock face protected from precipitation and wind erosion. The varnish likely originates from airborne dust and external surface runoff, including: clay minerals, oxides and hydroxides of manganese (Mn) and/or iron (Fe), sand grains, trace elements, and usually organic matter. Streaks of black varnish often occur where water cascades over cliffs protected from wind. Varnish color varies from shades of brown to black. Manganese-poor, iron-rich varnishes are red to orange, and intermediate concentrations are shaded brown. Manganese-oxidizing microbes may explain the unusually high concentration of manganese in black desert varnish, which can be smooth and shiny where densest.
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